Process for preparation of maleic anhydride

ABSTRACT

The present invention relates to a process for the regeneration and stabilization of certain phosphorus-vanadium-oxygen complex catalysts or phosphorus-vanadium-oxygen co-metal complex catalysts, with halogen-containing components and subsequently treating the catalyst with phosphorus compounds. These catalysts are useful for the manufacture of maleic anhydride from butane feedstock.

This is a division of application Ser. No. 150,223, filed Jan. 29, 1988now U.S. Pat. No. 4,861,738.

BACKGROUND FIELD OF THE INVENTION

The field of art to which this invention pertains is to regeneration andstabilization of phosphorus-vanadium-oxygen complex catalysts and, inparticular, the stabilization of catalysts used for the production ofoxygenated products including aldehydes, ketones, acids, anhydrides, andmixtures thereof from hydrocarbon feedstocks. In a specified instance,the claimed regeneration and stabilization procedure is advantageouslyused on a catalyst used for the production of maleic anhydride from aspecified feedstock, namely, normal butane in air. Pertinent U.S. Patentand Trademark Office classifications include Class 549, subclass 259.

BACKGROUND OF THE INVENTION

In general, catalysts proposed for the oxidation of butane, other C₄hydrocarbons, and benzene to maleic anhydride have been based uponvanadium and phosphorus. In U.S. Pat. No. 3,293,268, it is disclosedthat the oxidation of butane to maleic anhydride can be performed in thepresence of a phosphorus-vanadium-oxygen-containing complex catalyst.Though this catalyst is capable of oxidizing butane, it does not givesufficiently high yields of maleic anhydride. Yields of maleic anhydrideof only 30 to 50 weight percent are reported. Various activators,stabilizers, and promoters have been disclosed in the prior art toimprove the yields of maleic anhydride. References include U.S. Pat.Nos. 3,867,411; 3,832,359; 3,888,886; 4,002,650; 4,147,661; 4,149,992;4,151,116; 4,152,338; 4,152,339; 4,081,460; 4,043,943; 4,154,703; andBritish Application 2,019,839A. While the aforementioned prior art tendsto bring about some improvement in the performance of thephosphorus-vanadium catalyst, there remains much room for improvement,particularly from the standpoint of high conversion, yield, and catalystlife. U.S. Pat. No. 4,081,460 relates to molybdenum-vanadium catalystswhich are regenerated by the addition of phosphorus. Other references ofinterest include U.S. Pat. Nos. 4,002,174; 4,094,816; 4,089,807;3,296,282; 3,474,041; and British Patent 1,464,198.

SUMMARY OF THE INVENTION

Our invention is a process for the in situ regeneration andstabilization of aged hydrocarbon oxidation catalysts whose selectivityto maleic anhydride has declined. The aged catalyst is first treatedwith a carbon halide compound which removes the old surface of thecatalyst. A phosphorus compound is then added to the catalyst feedstream to generate the optimum P/V atomic ratio at the active catalystsites. A surprising feature of this invention is the combined use of twoentirely different regeneration processes to produce a catalyst having aperformance which is superior to that produced by either of theprocesses alone.

The selectivity of butane oxidation catalysts which comprise primarilyphosphorus and vanadium declines with time depending on feed rates andoperating temperatures. This decline in selectivity generally is causedby the loss of phosphorus from the catalyst surface leading to activesites which do not have an optimum P/V atomic ratio. This loss inselectivity leads to a higher feedstock variable cost, reducedproduction of maleic anhydride, and poorer product quality. To replacethis catalyst is very expensive in terms of the lost production whilethe unit is shut down, the labor required to change the catalyst, andmost importantly, the cost of the new catalyst which is very expensive.

If this catalyst can be regenerated and stabilized, the economic savingsare substantial. We disclose herein a novel process which regeneratesand stabilizes a butane oxidation catalyst which has declined inselectivity. Moreover, this process requires no unit modifications andminimal unit downtime. The yield of maleic anhydride obtained from acatalyst regenerated by our novel process is higher than the yieldobtained from the singular use of either the carbon halide regenerationor regeneration with phosphorus compounds.

In our novel catalyst regeneration process, the catalyst which hasdeclined in yield is first treated in situ with carbon halides such asCCl₄ to remove the catalyst surface. This is suitably carried out byadding the carbon halide to the feed stream of n-butane and air duringthe production of maleic anhydride or to a carrier gas such as nitrogenwhen the oxidation is discontinued. It is preferred to add the carbonhalide to nitrogen carrier gas which is passed over the catalyst bed.The temperature of the catalyst during the carbon halide treatmentshould be 300° C. to 550° C. Temperatures between 350° C. and 500° C.are preferred. The amount of carbon halide such as carbon tetrachlorideadded to the catalyst is suitably varied from about 0.005 g CCl₄ /gcatalyst to about 0.5 g CCl₄ /g catalyst depending upon the amount ofcatalyst surface removal desired. The preferred range for most catalystsis 0.01-0.1 g CCl₄ /g catalyst. The flow rate of carrier gas such asnitrogen or butane-air feed is also suitably varied from 100 to 3000VHSV depending on the concentration of carbon halide or CCl₄ desired inthe carrier gas or feed. A nitrogen flow rate of 100-1000 VHSV ispreferred to minimize nitrogen usage and to maintain a sufficientconcentration of carbon halide in the nitrogen stream. The concentrationof carbon halide feed or carrier gas stream are also a function of thecarbon halide addition rate. Addition times suitably range from 1-60minutes but times of 10-30 minutes are preferred. Practicalconcentrations of carbon halide in the feed or carrier gas are suitablyabout 1 to about 80 mole percent. Preferred concentrations of carbonhalide are about 5 to about 50 mole percent and are dependent on theflow rate of the feed or carrier gas and addition time of the carbonhalide to that stream.

Once the catalyst has been treated with carbon halide such as CCl₄ andthe catalyst has reactivated, phosphorus compounds such astriethylphosphate are added continuously to the butane-air feed streamto optimize the P/V ratio on the surface of the catalyst and increasethe yield of maleic anhydride from the catalyst. Other alkylphosphatessuch as trimethylphosphate, phosphites such as trimethylphosphite andtriethylphosphite, and phosphines are examples of phosphorus compoundsthat are usually added to the feed stream in place of triethylphosphate.These compounds are added with or without water, but the presence of asmall amount of water in the feed is preferred. The water present in theair feed to the reactor may be sufficient for good performance. Theamount of water added is about 1,000 parts per million to about 40,000parts per million by weight of the reactor feedgas stream. Feedstockssuch as butadiene and butenes may also be used in this invention toproduce maleic anhydride.

DETAILED DESCRIPTION OF THE INVENTION

A broad embodiment of this invention resides in a method forregenerating and stabilizing a phosphorus-vanadium-oxygen complexcatalyst having an atomic ratio of phosphorus to vanadium in the rangeof from about 0.5 to about 5, which method comprises contacting thecatalyst with an effective amount of a reactivating material selectedfrom the group consisting of: (1) molecular chlorine or fluorine, ormixtures thereof; (2) halides of fluorine, chlorine, bromine, or iodine,being in the vapor state above 250° C. at atmospheric pressure,represented by the following formula C(X)_(n), where each X is aselected halide and n is an integer from about 1 to 4, any remainingradicals being hydrogen or mixtures thereof; (3) organic halides offluorine, chlorine, bromine or iodine being in the vapor state aboveabout 250° C. at atmospheric pressure represented by the formulaR(X₁)_(m), where R is alkane, alkene or alkyne, of straight or branchedstructure, having at least two carbon atoms and X₁ is independently aprimary, secondary, or tertiary halide and m is an integer of from 1 toabout 20 consistent with the number of carbon atoms of said structure,or mixtures thereof; (4) hydrogen halides or mixtures thereof atreactivating conditions including a temperature in the range of fromabout 300° C. to about 550° C.

The catalyst is then further regenerated and stabilized by the additionof phosphorus compounds or mixtures thereof such as alkylphosphates,phosphites, and phosphines. This further regeneration and stabilizationtakes place at a temperature of about 300° C. to about 550° C.Representative phosphorus compounds have the following structure:##STR1## wherein R is a phenyl or an alkyl radical of 1 to 6 carbonatoms and X is H or R. Suitable compounds are primary, RPH₂, secondary,R₂ PH, and tertiary, R₃ P, phosphines; such as ethyl phosphine; thetertiary phosphine oxides, R₃ PO, such as tripropyl phosphine oxide; theprimary RP(O)(OX)₂, and secondary, R₂ P(O)OX, phosphonic acids, such asbenzene phosphonic acid; the esters of the phosphonic acids, such asdiethyl methane-phosphonate; the phosphonous acids, RPO₂ X₂, such asbenzenephosphonous acid and the esters thereof, such as the monoethylester; the phosphinous acids, R₂ POX, such as diethyl phosphinous acidand the esters thereof, such as the monoethyl ester; the primary,ROP(OX)₂, secondary, (RO)₂ POX, and tertiary, (RO)₃ P, phosphites, suchas diethyl phosphite, trimethyl phosphite, triethyl phosphite,triisopropyl phosphite, tripropyl phosphite and tributyl phosphite, andthe pyrophosphites, such as tetraethyl pyrophosphite. The preferredphosphorus compound is an ester of orthophosphoric acid having theformula (RO)₃ P═O wherein R is hydrogen or C₁ -C₄ alkyl, at least one Rbeing C₁ -C₄ alkyl. The preferred phosphorus compounds aretriethylphosphate or trimethylphosphate.

In another embodiment of this invention, the phosphorus-vanadium-oxygencomplex catalyst may include a cometal. Suitable cometals includemolybdenum, zinc, tungsten, uranium, titanium, zirconium, antimony,niobium, cobalt, chromium, tin, iron, manganese, nickel, or mixturesthereof.

The present regeneration and stabilization process is useable for manyphosphorus-vanadium-oxygen complex catalysts. In fact, these catalystscontain phosphorus, vanadium, and oxygen, and advantageously theyinclude other metals for activation or stabilization of the catalyst.

A representative list of catalysts which can be utilized to producemaleic anhydride from butane or aromatic materials or mixtures thereofare described in the following U.S. Pat. Nos., 3,867,411; 3,832,359;3,888,886; 4,002,650; 4,147,661; 4,149,992; 4,151,116; 4,152,338;4,152,339; 4,081,460; 4,043,943; 4,154,703.

The specific oxygenated product will depend on reaction conditions,feedstock selection and catalyst type utilized. Hydrocarbon feedstocksand the products produced therefrom include: ethane and products whichare produced from it generally of non-acidic materials; propane whichcan produce acrylic acid, acetic acid, maleic acid, and, in someinstances, propionic acid; normal butane, butenes or butadienes whichcan produce in certain instances, maleic anhydride and other productsincluding acetic acid, acrylic acid, and methyl acrylic acids; normalpentane, which can produce maleic anhydride and other materialsincluding formic acid and other trace materials; propylene, which can beused to produce maleic anhydride and other acid products, paraxylene,which can be used to produce maleic anhydride; orthoxylene, which can beused to produce phthalic anhydride and, in some cases, maleic anhydride;benzene, which can produce, in many instances, high concentrations ofmaleic anhydride and other acetic-based materials.

The catalysts contemplated for use in the claimed regeneration andstabilization process are generally made from the reaction of vanadiumpentoxide and phosphoric acid under controlled conditions. Othervanadium and phosphorus-containing materials can be used in the catalystpreparation. Various cometals may be added to the phosphorus-vanadiumoxide catalyst to improve activity or selectivity. Depending on the feedto be processed and the desired products, the composition of thecatalyst can be varied significantly.

For maleic anhydride production from normal butane, a suitable catalysthas an atomic ratio of phosphorus to vanadium of from about 0.5 to about5. An even more preferred ratio is a value of from around 1.0 to about1.6. Other metals may be incorporated into the basic catalyst in varyingratios of from about 0.001 to about 5 atoms of activator for each atomof vanadium. An especially useful catalyst for maleic anhydrideproduction from normal butane has an atomic ratio of phosphorus tovanadium to zinc of about 1.15:1.0:0.19.

The concentration of the regenerating agent passing over the catalystshould be monitored so as to prevent damage to the catalyst from excessadditions. Additional problems associated with regeneration agentadditions include the production of corrosive end products whichpossibly could damage plant equipment.

It has been found in determining what is an effective amount ofregeneration agent that there is some minimum concentration of theregeneration agent which should be passed into the reaction zone toeffect the increase in selectivity of the catalyst. However, it isdifficult to ascertain the concentration as an absolute quantity sincereactor designs would have a substantial influence on the actualconcentration to which the catalyst within the reaction zone would beexposed. Accordingly then, the better approach would be to state that aminimum total quantity based generally on the phosphorus and/or vanadiumcontent in the reaction zone be passed into the reaction zone forregeneration conditions to give the necessary selectivity increase.

Carrier gases are contemplated when the regeneration procedure occurs tomove the regeneration agent through the catalyst bed. The carrier gasesare not necessarily critical and can include materials such as nitrogen,butane, oxygen, or any other available gaseous stream which would becompatible with the regeneration agent and would not degrade thecatalyst performance.

Regeneration and stabilization conditions include a temperature in therange generally from about 300° C. to about 550° C. In a preferredinstance, the regeneration conditions suitably include a temperaturewithin the range of from about 350° C. to about 500° C., and in someinstances, from about 375° C. to about 475° C. Of course, thetemperatures of regeneration will vary depending on the specificcatalyst and oxidation process ultimately utilized. When normal butaneand air or enriched oxygen are passed into the reaction zone for theproduction of maleic anhydride, it has been found that a most preferredregeneration temperature will be somewhere above 350° C., but below 500°C. when a carbon tetrachloride regeneration agent is used in conjunctionwith a phosphorus compound.

For the most successful regeneration and stabilization of a butaneoxidation catalyst for producing maleic anhydride when using a carbontetrachloride regeneration agent in conjunction with a phosphoruscompound, it has been found that regeneration temperatures greater thanabout 375° C. are desired to cause increases in selectivity but lessthan about 475° C. are needed to reduce losses in catalyst conversion.

The halogen-containing regeneration agents which may be used in theregeneration and stabilization process disclosed herein generallyinclude materials such as molecular halogens or mixtures thereof, orcompounds containing one or more halide radicals or mixtures thereof.However, within the broad category of halides there obviously existmaterials with hazardous properties, e.g., self-detonation or highlycorrosive tendencies which, while within the definition of halides forregeneration agents, would not necessarily be effective since theydestroy the catalyst and/or the processing equipment. Accordingly then,in defining the regeneration agents or halides used herein, theinoperative species are to be precluded.

One of the basic requirements when utilizing the halide materials asregeneration agents is that they remain in a vapor phase when employedat reactivating conditions. Accordingly then, materials which havereasonably high boiling points are not practical and would presentprocessing difficulties. It is preferable that the halide materials bein a vapor phase at temperatures above a minimum of about 250° C. atatmospheric pressure. The specific regeneration agents can include purecomponents or mixtures of components. Specifically useful in theregeneration step herein are the halides including the gaseous forms offluorine, chlorine, and bromine. In some instances, gaseous iodine maybe used, but its boiling point is sufficiently high so that it may notpresent a favorable regeneration agent when used at low temperatures.Specific regeneration agents can include, but are not necessarilylimited to the following: hydrogen chloride, trichloromethane,dichloromethane, monochloromethane, hexachloroethane, halide-substitutedethanes, propanes, butanes (normal or iso), pentanes (normal orbranched), hexanes (branched or straight), and other chloride orhalide-containing aliphatics. Other specific halides which can beutilized include materials such as 1,6-dichlorohexane,1,2-dichlorohexane, 1,2-dibromohexane, 2,2-dichlorohexane,2,3-dichlorohexane, 2,5-dichlorohexane, and 3,4-dichlorohexane, normalhexylbromide, sec-hexylbromide and 3-bromohexane.

Organic halides of fairly low carbon number (generally 4 or less) arepreferred to reduce the possibility of coke formation duringregeneration.

Inter halogens which may be utilized include gases which have reasonablylow boiling points such as ClF, ClF₃, BrF, BrCl, IBr, BrF₅, F₂ O, Cl₂ OClO₂ (potentially explosive), Cl₂ O₆, Cl₂ O₇, Br₂ O and oxy acids ofchlorine, bromine, and iodine. Other materials which may be utilizableat high reactivation temperatures include products such as CF₄, CHF₃,Freon 12, Freon 13, Freon 22, Freon 21 and trichloroacetic acid.

One class of regeneration agents includes organic halides being in thevapor state above about 250° C. at atmospheric pressure represented bythe formula:

    C(X).sub.n

where each X is a selected halide and n is an integer from 1 to 4, anyremaining radicals being hydrogen. Carbon tetrachloride isrepresentative of the group and is especially preferred.

Another class of regeneration agents include organic halides being inthe vapor state above about 250° C. at atmospheric pressure representedby the formula:

    R(X.sub.1).sub.m

where R is alkane, alkene or alkyne of straight or branched structurehaving at least two carbon atoms and X₁ is independently a primary,secondary or tertiary halide and m is an integer of from 1 to about 20consistent with the number of carbon atoms of said structure.

This invention also comprises a process for oxidizing butane to maleicanhydride with the regenerated and stabilized catalyst wherein inaddition to the carbon halide treatment the catalyst has been treatedwith a phosphorus compound, particularly an alkyl ester oforthophosphoric acid, in the presence of about 1,000 to about 40,000parts per million by weight of water based on the total weight of thefeed gas stream. Generally, the amount of alkyl ester added is about 0.1to about 100,000 parts per million by weight of the reactor feed gasstream. In a preferred mode the amount of alkyl phosphate added is inthe range of about 0.1 to about 30 parts per million by weight of thereactor feed stream. Higher concentrations of alkyl phosphate generallyabove about 30 parts per million by weight are useful in a batchcatalyst regeneration process, preferably in a range of about 50 toabout 100,000 parts per million by weight of reactor feed gas stream andmore preferably about 1,000 to about 100,000 parts per million by weightof reactor feed gas stream. The regeneration and stabilization isconducted at a temperature of about 300° C. to about 550° F. The alkylphosphate in a water medium comprising about 0.001 to about 90 weightpercent, more preferably about 0.01 to about 50 weight percent, of thesolution is contacted with the feed gas stream flowing to the reactor.If desired, the water and alkyl phosphate may be added separately to thefeed gas stream instead of as a solution. Alternatively, the alkylphosphate and water may be added directly to the butane feed prior tothe mixing of the butane and air reactants. The oxidation of butane tomaleic anhydride may be accomplished by contacting n-butane in lowconcentration in oxygen with the described catalyst. Air is entirelysatisfactory as a source of oxygen, but synthetic mixtures of oxygen anddiluent gases such as nitrogen may also be employed. Air enriched withoxygen may be used.

The gaseous feed stream to the oxidation reactors will normally containair and about 0.2 to about 1.7 mole percent of the hydrocarbon such asbenzene, butane, butene or butadiene. About 0.8 to about 1.5 molepercent of the hydrocarbon is satisfactory for optimum yield of maleicanhydride for the process of this invention. Although higherconcentrations may be employed, explosive hazards may be encountered.Lower concentrations of the hydrocarbon feedstock, less than about onepercent, of course, will reduce the total yield obtained at equivalentflow rates and, thus, are not normally employed for economic reasons.The flow rate of the gaseous stream through the reactor may be variedwithin rather wide limits, but the preferred range of operations is atthe rate of about 100 to about 4000 cc of feed per cc of catalyst perhour and more preferably about 1000 to about 2400 cc of feed per cc ofcatalyst per hour. Lower flow rates make the butane oxidation processuneconomical. A catalyst should be effective at flow rates of about 1200to about 2400 cc of hydrocarbon feed per cc of catalyst per hour. Thereare catalysts which show good promise but when subjected to the hourlyspace velocity designated above show very poor yields. The amount ofwater added isabout 1,000 to about 40,000 parts per million by weight ofthe reactor feed gas stream. The preferred amount of water added isabout 5,000 to about 35,000 parts per million by weight of the reactorfeed gas stream. Residence times of the gas stream will normally be lessthan about four seconds, more preferably less than about one second, anddown to a rate where less efficient operations are obtained. The flowrates and residence times are calculated at standard conditions of 760mm of mercury and at 0° C.

The reaction may be conducted at atmospheric, super-atmospheric, orbelow atmospheric pressure. The exit pressure will be at least slightlyhigher than the ambient pressure to ensure a positive flow from thereactor. The pressure of the inert gases must be sufficiently high toovercome the pressure drop through the reactor.

A variety of reactors will be found to be useful, and multiple tube heatexchanger-type reactors are quite satisfactory. The tops of suchreactors may vary in diameter from about one-quarter inch to about threeinches, and the length may be varied from about three to about ten ormore feet. The oxidation reaction is an exothermic reaction and,therefore, relatively close control of the reaction temperatures shouldbe maintained. It is desirable to have the surface of the reactors atrelatively constant temperatures, and some medium to conduct heat fromthe reactors is necessary to aid temperature control. Such media may beWoods metal, molten sulphur, mercury, molten lead and the like, but ithas been found that eutectic salt baths are completely satisfactory. Onesuch salt bath is a sodium nitrate, sodium nitrite, potassium nitrateeutectic constant temperature mixture. An additional method oftemperature control is to use a metal block reactor whereby the metalsurrounding the tube acts as a temperature regulating body. As will berecognized by one skilled in the art, the heat exchanger medium issuitably kept at the proper temperature by heat exchangers and the like.The reactor or reaction tubes are suitably iron, stainless steel, carbonsteel, nickel, glass tubes, such as vycor and the like. Both carbonsteel and nickel tubes have excellent long life under the conditions ofthe reaction described herein. Normally, the reactors contain a preheatzone under an inert material such as one-quarter inch alundum pellets,inert ceramic balls, nickel balls, or chips and the like present atabout one-half to one-tenth the volume of the active catalyst present.

The temperature of reaction may be varied within some limits, butnormally the reaction should be conducted at a temperature within arather critical range. The oxidation reaction is exothermic and oncereaction is underway, the main purpose of the salt bath or other mediais to conduct heat away from the walls of the reactor and control thereaction. Better operations are normally obtained when the reactiontemperature employed is no greater than 20°-50° F. above the salt bathtemperature. The temperature of the reactor, of course, will also dependto some extent upon the size of the reactor and the butaneconcentration.

Maleic anhydride may be recovered by a number of ways well-known tothose skilled in the art. For example, the recovery may be by directcondensation or by absorption in suitable media, with specificoperations and purification of the maleic anhydride.

In order to more adequately understand and describe the novelregeneration and stabilization process the following definition of termsis presented. ##EQU1##

In a specific instance wherein a feed stream containing essentiallynormal butane is charged to the reaction zone for the production ofmaleic anhydride, the conversion, selectivity and mole yield are shownbelow. ##EQU2##

In instances in which a weight yield is desired for the production ofmaleic anhydride from normal butane, the following calculation can beused.

    Weight Yield=(Conversion)(Selectivity)(1.69)

The above conversion, selectivity and yields on the molar basis times100 equal percentage conversion, selectivity and mole yields. Whendetermining a weight yield, it is necessary to know the ratio of themolecular weights of the feed hydrocarbon and the oxygenation productand, accordingly, the weight yield for the production of maleicanhydride from normal buane is defined as the product of the molarconversion times the molar selectivity (for normal butane to maleicanhydride), all times 1.69. The theoretical maximum production of maleicanhydride from normal butane would give a weight yield of 1.69 pounds ofmaleic anhydride for each pound of normal butane consumed, assuming 100percent selectivity and conversion. In stating the weight yield on apercentage basis, it merely reflects the quantity of theoretical weightyield of maleic anhydride times 100. Accordingly then, the theoreticalweight percent yield would be 169 percent.

The following examples are presented to specifically illustrate certainembodiments of the claimed regeneration and stabilization processherein; and are not necessarily presented so as to unduly limit orrestrict the scope of the claims:

EXAMPLES

The maleic anhydride yield of a phosphorus-vanadium-oxygen catalyst in apilot plant having a 33 inch catalyst bed and a 0.62 inch i.d. reactordeclined to 59 wt.% at a salt bath temperature of 817° F., 1.5% n-butanein air feed, and 2000 VHSV after 3832 hours on stream. At this time, thefeed stream was directed through a saturator containing an aqueoussolution of triethylphosphate. During the next 5797 hours on stream, thecatalyst yield at 1.5% n-butane in air and 2000 VHSV was maintained at61-69 wt.% using aqueous triethylphosphate solutions in the feed gassaturator ranging in concentration from 1 g triethylphosphate/1 of H₂ Oto 22 g triethylphosphate/1 of H₂ O.

The triethylphosphate addition was then discontinued and the yielddeclined to 59 wt.% at 816° F. and the same n-butane concentration andflow rate after 9821 hours. The feed to the reactor was thendiscontinued and nitrogen at about 100 VHSV was passed over the catalystat 810° F. CCl₄, 13 g, was added to the nitrogen stream in 15 minutesusing a syringe pump. At 9965 hours, the yield of the catalyst was 64wt.% at 821° F., 15% n-butane, and 2000 VHSV. The feed was passedthrough a saturator containing an aqueous solution of triethylphosphatewith a concentration of 2-6 g triethylphosphate/1 of H₂ O. A maximummaleic anhydride yield of 69 wt.% was achieved at 10156 hours. However,the yield declined to 65 wt.% at 10493 hours.

At 10517 hours, the triethylphosphate addition was again discontinued.At 834° F., 2000 VHSV, and 1.5% n-butane in the feed, 13 g of CCl₄ wereinjected into the feed stream using a syringe pump. The maleic anhydrideyield was only 53 wt.% at 10613 hours when the feed stream was again putthrough a saturator containing 4 g of triethylphosphate/1 of H₂ O. Theyield improved to 69 wt.% at 837° F. and the same flow conditions after10776 hours. A maximum maleic anhydride yield of 72 wt.% was achievedafter 11160 hours at 854° F. with a saturator concentration of 8 gtriethylphosphate/1 of H₂ O. This yield was sustained for a period ofone week demonstrating that this improved yield was not an excursion inthe data.

This example shows that the use of CCl₄ followed by the addition oftriethylphosphate and water improves the yield of an aged catalyst overthe singular use of either method. Triethylphosphate and water treatmentalone gave a maximum maleic anhydride yield of 69 wt.%. Treatment withCCl₄ followed by the addition of triethylphosphate and water improvedthe yield of 72 wt.% which is a 3 wt.% increase in yield.

I claim:
 1. A catalytic process for the oxidation of n-butane to maleic anhydride wherein n-butane is contacted with a catalytic amount of vanadium phosphorus oxide catalyst having an atomic ratio of phosphorus to vanadium in the range of about 0.5 to about 5 regenerated according to the following process comprising:(A) contacting said catalyst at a temperature in the range of from about 300° C. to about 550° C. with an effective amount of a material selected from the group consisting of:(1) molecular chlorine of fluorine or mixtures thereof; (2) carbon halides of fluorine, chlorine, bromine or iodine being in the vapor state above about 250° C. at atmospheric pressure represented by the following formula:

    C(X).sub.n

where each X is a selected halide and n is an integer from 1 to 4, any remaining radicals being hydrogen or mixtures thereof; (3) organic halides of fluorine, chlorine, bromine or iodine being in the vapor state above about 250° F. at atmospheric pressure represented by the formula;

    R(X.sub.1).sub.m

where R is alkane, alkene or alkyne of straight or branched structure having at least two carbon atoms and X₁ is independently a primary, secondary or tertiary halide and m is an integer of from about 1 to about 20 consistent with the number of carbon atoms of said structure or mixtures; (4) hydrogen halides or mixturs thereof at regeneration conditions including a temperature in the range of from about 300° C. to about 550° C.; and (B) contacting the catalyst with an effective amount of an alkyl ester of other orthophosphoric acid and water.
 2. The process of claim 1 wherein the catalyst is regenerated by contacting it during the oxidation with water and the phosphorus compound, wherein the amount of water added is about 1,000 parts per million to about 40,000 parts per million by weight of the reaction feedstock.
 3. A catalytic process for the oxidation of n-butane to maleic anhydride wherein n-butane is contacted with a catalytic amount of vanadium-phosphorus-oxide-cometal catalyst having an atomic ratio of phosphorus to vanadium in the range of about 0.5 to about 5 comprising:(A) contacting said catalysft at a temperature in the range of from about 300° C. to about 550° C. with an effective amount of a material selected from the group consisting of:(1) molecular chlorine or fluorine or mixtures thereof; (2) carbon halides of fluorine, chlorine, bromine or iodine being in the vapor state above about 250° C. at atmospheric pressure represented by the following formula:

    C(X).sub.n

where each X is a selected halide and n is an integer from 1 to 4, any remaining radicals being hydrogen or mixtures thereof; (3) organic halides of fluorine, chlorine, bromine or iodine being in the vapor state above about 250° C. at atomospheric pressure represented by the formula:

    R(X.sub.1).sub.m

where R is alkane, alkene or alkyne of straight or branched structure having at least two carbon atoms and X₁ is independently a primary, secondary or tertiary halide and m is an interger of from about 1 to about 20 consistent with the number of carbon atoms of said structure or mixtures; (4) hydrogen halides or mixtures thereof at regeneration conditions including a temperature in the range of from about 300° C. to about 550° C.; and (B) contacting the catalyst with an effective amount of an alkyl ester of orthophosphoric acid and water.
 4. The process of claim 3 wherein the cometal is selected form the group of zinc and molybdenum.
 5. The process of claim 4 wherein the catalyst is regenerated according to claim
 2. 6. The process of claim 1 wherein the oxidation is conducted at a temperature of about 700° F. to about 900° F.
 7. The process of claim 3 wherein the oxidation is conducted at a temperature of about 700° F. to about 900° F.
 8. The process of claim 4 wherein the oxidation is conducted at a temperature of about 700° F. to about 900° F.
 9. The process of claim 3 wherein the cometal is selected from the group of zinc, bismuth, copper, molybdenum, lithium, tungsten, chromium, uranium, niobium, zirconium, tin, cobalt, iron, nickel, antimony, titanium, or mixtures thereof.
 10. The process of claim 9 wherein the oxidation is conducted at a temperature of about 700° F. to about 900° F. 